The present invention relates to a system or instrument for computing blood flow rates for vascular systems. In particular, the present invention employs what is referred to in the art as "thermodilution" techniques.
The principle upon which the thermodilution technique is based is that the change of heat of a substance is related to its mass and specific heat for a given change in temperature. For a static system, if two substances at different temperatures are mixed, the resulting temperature of the mixture will fall between the starting temperatures of the two substances. If the mass of one substance is unknown, it can be determined by equating at equilibrium the change in heat of the two substances and calculating the unknown mass from the resulting equation.
When this principle is applied to a system of continuous flow, as in the heart and vasculature, a small amount of relatively cool substance (called the "indicator") is introduced into and mixed with the flowing substance (i.e., the blood), thereby yielding a time-temperature curve which may be sensed slightly downstream of the point at which the indicator is introduced into the system. This curve is referred to as the "thermodilution curve", and the area under the thermodilution curve represents the sum of the instantaneous mixed temperatures at the sensing point. The well-known Stewart-Hamilton equation relates the unknown flow of mass per unit time to the change of heat in the mixture, the specific heat, and the change in temperature.
It is desired that the amount of the indicator introduced into the bloodstream is small relative to the mass of body tissue (including blood).
The thermodilution technique for determining cardiac output flow utilizes the Stewart-Hamilton indicator dilution equation as modified for a thermal indicator, as follows: ##EQU1## Where:
______________________________________ C.O. = Cardiac output in Liters/Minute This is the ratio of the density times the .rho.C.rho. (5 % Dextrose) 1.08 = specific heat of 5 % .rho.C.rho.(Blood) Dextrose to the density times the specific heat of blood. C.sub.T = Correction factor for the injectate temperature rise through the catheter 60 = Seconds/Minute V.sub.1 = Volume of injectate in liters T.sub.B = Initial blood temperature in .degree. C T.sub.1 = Initial injectate temperature in .degree. C .infin. .intg. .DELTA.T.sub.B (t) dt = Area under the thermodilution o curve in .degree. C-Sec. ______________________________________
The first four terms of Equation (1), namely [(1.08 C.sub.T) 60 V.sub.I ]are grouped together and entered into the system of the present invention as a preset constant value, called the "computation constant". The temperature of the blood, T.sub.B, and the initial temperature of the indicator or injectate, T.sub.I, are separately entered in a similar manner. All of the entries of constants are made prior to initiating the test-- that is, prior to generating the thermodilution curve.
The present system determines the difference between T.sub.B and T.sub.I and multiplies this difference by the computation constant. The resulting value, sometimes referred to as the "numerator value" of the equation is internally stored for later processing. To conduct a test, a catheter is inserted into the pulmonary artery. The catheter also contains a port spaced from the thermistor for introducing a known amount of indicator at a predetermined temperature. A sensing thermistor is located on the catheter, and the catheter is inserted in such a manner that the sensor is located downstream in the direction of blood flow from the point of injection of the indicator. Hence, the thermistor senses the temperature of the blood at a location downstream of the point of injection of the indicator, and generates a time-temperature thermodilution signal which is then amplified and integrated. Integration of the thermodilution signal is terminated after the curve has peaked and begins to diminish to a predetermined value equal to 30% of its peak value. The total integral is then estimated by increasing the value of the truncated integral by a fixed percentage. Studies have shown that if the integration is carried out until the curve returns to 30% of its peak value, then the truncated integral value must be increased by 22% to estimate the remaining portion of "tail" of the integral. The resulting value is sometimes referred to as the "estimated integral value". The amount by which the truncated integral value is increased to determine the estimated integral value is, of course, dependent upon the point at which the integration terminates. As mentioned, in the preferred embodiment, where the integration is carried out to a point at which the thermodilution curve returns to a value equal to 30% of its peak value, then the value of the truncated integral must be enhanced by 22%. The increase of the truncated integral value may be accomplished either by increasing the gain of an amplifier to be 1.22 greater than unity gain, or there may be a separate computation of 22% of the truncated integral value which would then be added to the truncated integral value.
Statistical analysis of over 200 thermodilution procedures indicates that measurement error is minimized if the integration is terminated when the curve returns to 30% of its peak value. The ratio of the numerator value stored in the system and described above to the estimated integral value is then computed in a dual-slope ratiometric analog-to-digital converter to compute the final value of cardiac output flow.
Systems are known for computing cardiac flow rates, such as those discussed and the one claimed in U.S. Pat. No. 3,678,922, granted July 25, 1972, to Philips et al. In the latter system, integration of a dilution curve is terminated at a cutoff point which is defined in terms of the peak of the response signal. The cutoff point of the response signal is determined to compensate for error normally introduced due to recirculation of the indicator. Recirculation occurs when the indicator passes the sensor a second time so as to obscure the true reading of the integral of the thermodilution curve. Although recirculation may cause measurement errors for a dye dilution procedure, (since the effect of the dye is not readily diminished), no substantial recirculation problems have been experienced in the case of thermodilution measurements. Nevertheless, it is desirable to terminate integration of the thermodilution curve before the curve approaches the baseline to avoid other end artifacts such as: fluctuation of the baseline temperature in the pulmonary artery caused by respiration, and significant loss of indicator in patients with pulmonary congestion, which may result in falsely high measurements.
The particular features and advantages of the present invention can best be appreciated from a detailed understanding of the system as described in the following detailed disclosure accompanied by the attached drawing wherein identical reference numerals will refer to like parts in the various views.